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Constraints on Earth accretion deduced from noble metals in the oceanic mantle

机译：由海洋地幔中的贵金属推断出的对地球积聚的限制

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摘要

If the Earth's mantle were in equilibrium with its core, the mantle would contain three orders of magnitude less of the noble metals (platinum-group elements Pt, Os, Ir, Ru, Pd and Rh, plus Au and Re) than are observed. An explanation put forward to account for this disparity has been that the last 1% of the Earth's accretion occurred after the iron-rich core had separated from the mantle. Recent debate has accordingly centred on which meteorite class or classes made up this 'late veneer' of accretion. Here we present analyses of noble-metal concentrations in oceanic peridotites (plutonic rocks which are thought to represent samples of the Earth's upper mantle). We find that the average oceanic-mantle Os/Ir ratio is indistinguishable from that in the CI-type carbonaceous chondrites, but that Ru/Ir, Pt/Ir, Rh/Ir and Pd/Ir ratios are about 40% higher. A late veneer composed of strictly CI-type carbonaceous chondritic composition is therefore not compatible with these observations. The data also allows us to rule out other carbonaceous chondrites, enstatite chondrites and ordinary chondrites as significant late veneer components. We propose that mixing of differentiated outer-core material back into the mantle after core separation could account for the observed noble-metal ratios and abundances in the mantle without any late accretionary veneer.
机译：如果地球的地幔与其核心处于平衡状态，那么地幔将比所观测到的贵金属（铂族元素Pt，Os，Ir，Ru，Pd和Rh以及Au和Re）少三个数量级。提出解释这种差异的一种解释是，地球上积聚的最后1％发生在富铁核心与地幔分离之后。因此，最近的辩论集中在哪个或哪些陨石类构成了这种“单板”积聚。在这里，我们对海洋橄榄岩（被认为是代表地球上地幔样品的深成岩）中的贵金属浓度进行了分析。我们发现海洋地幔的Os / Ir比值与CI型碳质球粒陨石的平均比值没有区别，但是Ru / Ir，Pt / Ir，Rh / Ir和Pd / Ir比值高约40％。因此，由严格CI型碳质软骨成分组成的后期胶合板与这些观察结果不兼容。数据还使我们可以排除其他碳质球粒陨石，顽辉陨石球粒陨石和普通球粒陨石作为重要的后期胶合板成分。我们建议，在芯分离后，将分化的外芯材料混合回地幔中，可以解决观察到的地幔中贵金属比和丰度的问题，而无需后期增生饰面。

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  • 来源
    《Nature》 |1998年第6663期|p.166-169|共4页
  • 作者

    Jonathan E. Snow; Gerhard Schmidt;

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  • 收录信息 美国《科学引文索引》(SCI);美国《工程索引》(EI);美国《生物学医学文摘》(MEDLINE);美国《化学文摘》(CA);
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类 自然科学总论;
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